Project Summary Stress is a known risk factor for clinical depression, and stress can exacerbate depressive symptoms in vulnerable individuals. By better understanding the molecular mechanisms by which stress can drive pro-depressive behaviors, we will be able to generate novel/tailored therapeutics to combat this prevalent disease. Both the kappa-opioid receptor (KOR) and mammalian target of rapamycin (mTOR) systems have been shown to mediate elements of stress-induced, depression-like behaviors in mice. However, these two systems have never been linked, representing a major gap in our knowledge of a complete pathway from stress to pathophysiology and behavior. Understanding how KOR activity is related to mTOR function will be critical in refining drugs that target these systems. One possible intermediate protein in these pathways is p38 MAPK, which is activated by KOR and whose activity is necessary for stress-induced immobility, aversion, and social avoidance. The proposed experiments will directly test the relationship between KOR, p38 MAPK, and mTOR as a functional signaling pathway mediating depression-like behaviors and neuronal plasticity. Specifically, we propose to first validate preliminary data demonstrating that mTOR levels are modulated by KOR activation. In Aim 1, we will use pharmacological inhibitors/antagonists and genetic models to test whether p38 MAPK is necessary for the changes in mTOR level seen by KOR activation. We will then assess whether mTOR mediates KOR/p38-induced place aversion and changes in swim stress immobility. In aim 2, we will test whether KOR changes markers of neuronal plasticity including dendrite length, spine density/shape, and synaptic proteins. We will use both pharmacological activation of KOR and stress to test dependence of p38 MAPK and mTOR, and finally determine if KOR antagonism is able to attenuate changes in stress-induced plasticity. The proposed studies described here will better define the molecular pathways from stress exposure to depression-like behaviors, and possibly uncover novel targets for therapeutic action that could be used in the clinical population to treat stress-vulnerable individuals.